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Guests & Members Post & Read => Open Forum => Topic started by: rlc on May 13, 2019, 12:21:23 PM

Title: Hamrak Mast & Runner Change Proposals
Post by: rlc on May 13, 2019, 12:21:23 PM
Please use this forum for discussion concerning the specification changes suggested by Peter Hamrak that appear in the May issue of Runner Tracks.  This is a neutral forum that is not sponsored by the author of the proposals as opposed to the one suggested in the runner Tracks article.  Furthermore please be aware that the proposals as they appear Runner Tracks will not likely be the final version of the proposals.  The proposals must first be reviewed by the Technical Committee, and they may edit those proposals in any manner they choose.  They can also make recommendations with regard to those proposals before they are voted on, but we will be voting on them.  Peter Hamrak is on the Technical Committee.  A robust discussion concerning the runner and mast change proposals will be of benefit to everyone.  Further commentary on the other two proposals that were rejected is also welcome. US3433

Title: Re: Hamrak Mast & Runner Change Proposals
Post by: wnethercote on May 13, 2019, 05:58:31 PM
Bob, here is what I recently posted to Peter's forum on masts.  I also did a runner post, to follow.

"I am following up on my Runner Tracks post, with a little additional detail.

I am not convinced that deleting current mast weight and centre of gravity restrictions would be performance neutral.  If I assume a 2.2Kg ballast weight at hound height in a 4.6 Kg mast I can derive a centre of gravity height for that 4.6 Kg mast by assuming that the ballasted mast had the minimum legal CG height.  The result is a CG height of 1604 mm for the 4.6 Kg unballasted mast.  Figure 1 shows this CG and the minimum legal CG.  Both gravity vectors point near the leeward end of the runner plank, about 200 and 400mm from its end, respectively.

The mast weights create a restoring moment that opposes the sail's capsizing force.  The unballasted mast is only 65% of the weight of the ballasted mast, but its gravity vector is twice as far from the leeward runner, so the lighter, lower-CG mast has 130% of the righting moment of the heavier mast.  In lighter winds, the mast will bend less and the righting arms of the two masts will be similar, so in lighter winds the heavier mast should have higher righting moments.  But in lighter winds, the lighter mast will be more desirable for weight reasons alone.

I don't think the differences are big; indeed, in Figure 1 the lighter mast only has about 4 or 5 N-m better righting moment.  What is of greater concern is shown in Figure 2.  The righting moments don't really act around the leeward runner, but around the line between the leeward runner and the bow runner.  The mast centre of gravity might actually be to leeward of that line in Figure 1, in which case there is even a bigger advantage for the lighter mast.

The mast centre of gravity rule is a pain in the neck without doubt.  Are the advantages of a lighter mast that I claim worth worrying about?  Perhaps not, but if the perception is that the advantages are significant then dropping these requirements may have negative effects on current mast values."

Peter has responded there. My estimates of ballast at the hound is on the high side.

Title: Re: Hamrak Mast & Runner Change Proposals
Post by: wnethercote on May 13, 2019, 06:02:21 PM
And my submission to peter's forum on runners:

"Hello Peter,

I am following up on my Runner Tracks article.

First I am solidly behind your goal of making our technical specifications clearer to users, and of making enforcement of technical specifications practical without resort to cutting up runners or sending them away for non-destructive testing (such as radiography or ultrasonic survey).

I am uncertain about making the construction of insert runner bodies totally free. As a home builder I would take advantage of it by using a thinner wood core and increasing my carbon fiber stiffening thickness. I do not think that I would ever build a foam-cored runner body. But I am concerned that allowing such a change would have a negative effect on the market value of current runners. The rules change would not affect their legality, as you say, but it might affect their resale value if newer runners were ween to be 'better' in some way.

Warren Nethercote, KC-3786"

Peter has provided responses to both my submissions.